Tech Tips

Motorcycle Charging Systems

Written by  January 31, 2009

When it comes to this subject you must ask yourself what is involved in my bike’s charging system. Let me clarify this to the best of my ability. Since 1978 to the present, motorcycles have what they call non-serviceable type charging systems. What does this mean to the average rider? This is a maintenance-free type system; however we are going into this a little deeper. You must consider that if you add up all the parts in today’s motorcycle charging systems, there are just three parts to make your ride event free.

1st component – Battery
The battery is the most critical part in starting the charging system. When you turn on the key, you are using stored chemical energy from the 12-volt battery; that is the hub of the motorcycle. Without a properly charged battery, this system will not function correctly. Battery voltage must remain constant to keep this system’s output at its peak. Throughout my tour of duty in the service industry, this is by far the most overlooked component in the charging system.

2nd component - Stationary Alternator
The alternator uses the principle of electromagnetism to produce voltage and current. A rotating magnet, called a rotor is located on the end of the crankshaft or alternator shaft. The strength and polarity of the voltage is dependent on the direction of rotation, the strength of the magnetic field, the number of conductors and the speed of the rotor inside the stator. This type of generator puts out AC voltage, which after the fact will change the flow from AC to DC current. To convert the AC voltage into DC voltage, a series of diodes are used. A diode is an electrical one-way check valve that allows current to flow in only one direction unless it is damaged. By allowing current to flow in only one direction, we are able to convert alternating current to direct current. This is a very essential requirement - to be in sync or you will have problems that cause costly repairs and LEAVE YOU STRANDED. The alternators on most current motorcycles are very efficient and produce incredible amounts of AC voltage. The key is making sure this voltage gets to the next piece in the charging system without getting any outside interference. Once the AC voltage has been generated from the Stationary Alternator, it will be sent to the Regulator/Rectifier. This is normally a finned-type part that is bolted directly to the frame. If you are riding today’s Harley, they are located directly behind the front fender on the frame support. If you are riding a metric cruiser/sport bike, they are usually closer to the battery. This makes for easier removal and repair.

3rd component - Regulator/Rectifier
The voltage regulator controls current applied to the alternator. When there is no current applied to the field, there is no voltage produced from the alternator. When voltage drops below 13.5, the regulator will apply current to the field and the alternator will start charging. When the voltage exceeds 14.5, the regulator will stop supplying voltage to the field and the alternator will stop charging. This is how voltage output from the alternator is regulated. Amperage or current is regulated by the state of charge of the battery. When the battery is weak, the electromotive force (voltage) is not strong enough to hold back the current from the alternator trying to recharge the battery. As the battery reaches a state of full charge, the electromotive force becomes strong enough to oppose the current flow from the alternator; the amperage output from the alternator will drop to 0, while the voltage remains at 13.5 to 14.5. When more electrical power is needed, the electromotive force will reduce and alternator amperage will increase. It is extremely important that when alternator efficiency is checked, both voltage and amperage outputs are checked also. Each alternator has a fixed amperage output depending on the electrical requirement of the motorcycle.

One of the best visual tests of your charging system is to rev up the motorcycle to where you cruise in high gear. If you have your bike inside, you can see the headlight get brighter when you rev the engine. Usually this happens around 2500-3500 rpm. This is a pretty great visual test to see if the charging system is reaching what we techs call the break-away voltage. I want to stress, this is NOT the actual procedure on a charging system test. This is strictly a visual inspection only. I recently had one of my customers’ Kawasaki in for a charging and starting problem. The regulator side of the regulator/rectifier had shorted and the alternator produced enough voltage to blow the main fuse and burn out the ignition module of his bike. Both of these components are very expensive and it left him stranded. We are all susceptible of this failure if we do not have the charging system tested periodically and have the battery in our motorcycle serviced on a routine schedule. Check the water level and charging on a trickle charger when you are not riding for three weeks or longer, there could be complications. Today’s systems are pretty bulletproof! Take the time and check your battery water level and if low, ONLY use distilled water. By adding tap water, the cells in the battery (6) will tend to sulfate faster. Distilled water has the minerals removed and you can buy this for $1 a gallon at your local grocery store.

Below are views of a Rotor, Stator and Regulator/Rectifier.

Be safe out there, and check your battery often.

Dave Miller